The tight U.S. Senate race in Massachusetts is getting feistier. Republican Sen. Scott Brown is going on the offensive, running his first attack ad against his Democratic challenger, Elizabeth Warren.

Yet going negative is risky, thanks to a pledge between the two candidates to keep out third-party attack ads.

A Brown TV ad that began airing Monday attacks Warren on an old issue in this race — how Warren identified herself as Native American during her academic career.

The incumbent senator debuted his more combative tone last Thursday in their first televised debate.

"Professor Warren claimed that she was a Native American, a person of color. And as you can see, she's not," Brown said, calling on his opponent to release her employment records.

At a Brown campaign stop over the weekend, supporters were filmed imitating their version of what has been called the "tomahawk chop." A blog called the "Blue Mass Group" posted a clip of the rally online, with the description, "Scott Brown staffers do 'Indian war whoop'"

Brown said Tuesday he doesn't condone that action during the rally, but that the real offense is that Warren claimed minority status. The Brown campaign has declined to say whether any campaign staffers were involved.

"At least two Republican staffers, including a member of Scott Brown's U.S. Senate office, apparently mocked Elizabeth Warren's claims to Native American ancestry by making tomahawk chops and war whoops outside one of Brown's campaign events in Dorchester, according to a video filmed by the state Democratic Party. ..."

"The state Democratic Party identified the staffers as Brad Garrett, who works for the Massachusetts Republican Party, and Jack Richard, a constituent services lawyer in Brown's U.S. Senate office."

In her own ad released this week, Warren is sticking to the story she says her family told her growing up — that her mother was part Native American.

"As a kid I never asked my mom for documentation when she talked about our Native American heritage. What kid would?" Warren asks. "But I knew my father's family didn't like that she was part Cherokee and part Delaware. So my parents had to elope."

Warren also says her ethnic background was never a consideration during her academic tenure.

"Let me be clear. I never asked for, never got any benefit because of my heritage. The people who hired me have all said they didn't even know about it," Warren says in her response ad.

"The voters liked him as sort of their friendly neighbor. And now, if he is a politician sort of wielding a knife, that is not going to be good for his image," says Berkovitz.

Brown's image as a regular guy who drives a pickup truck helped him win a stunning upset two years ago.

"Personally, he seems like a nice guy, seems like a good guy. You know, the kind of guy you'd hang out with," says Michael McGrath, a Boston Democrat who plans to vote for Warren.

But so far, he's not blaming Brown for going negative.

"It's a political race; anything goes," says McGrath. "It's the nature of the beast."

Joan Hoffman, a Brown supporter from Boston, points out that Warren aired the first attack ad in this race.

"If he continues to play the good guy, and she keeps throwing these barbs at him, he's gonna show a sign of weakness," says Hoffman. "He does have to defend himself."

Still, if Brown continues to launch personal attacks against Warren for the next six weeks, voters may not be so forgiving. Especially in a race where both candidates signed what they called the "People's Pledge" to keep often vicious, third-party ads out. Earlier this year, Brown boasted that the race was "serving as a model for the rest of the nation."

But Republican political analyst Todd Domke says it's not stopping the negativity.

"When the People's Pledge was first announced, people had higher expectations about this being an issues kind of contest. But now it will look more like a typical campaign," Domke says.

The agreement may not change the tone of the race. But Domke says at least it holds the candidates accountable for it.

While President Obama and Mitt Romney kept things pretty friendly there in New York, the U.S. Senate race in Massachusetts has taken a nasty turn. Republican Senator Scott Brown began running his first attack ad against Democratic challenger Elizabeth Warren.

Going negative is a risky move for Brown. Earlier, both candidates pledged to keep out third-party ads, so that means this new attack has to come with Brown's name on it. Curt Nickisch has the story from member station WBUR in Boston.

(SOUNDBITE OF CAMPAIGN AD)

SENATOR SCOTT BROWN: I'm Scott Brown, and I approve this message.

CURT NICKISCH, BYLINE: The new TV ad from Senator Scott Brown attacks his opponent on an old issue in this race: how Elizabeth Warren identified herself as Native American during her academic career.

(SOUNDBITE OF TV AD)

UNIDENTIFIED WOMAN: She's facing tough questions about whether she claimed to be a minority for professional gain.

UNIDENTIFIED MAN #1: Warren did give an answer. The problem is...

NICKISCH: Scott Brown debuted his more combative tone a few days earlier in their first televised debate. The senator confronted his blonde-haired opponent from the very outset.

(SOUNDBITE OF DEBATE)

BROWN: Professor Warren claimed that she was a Native American, a person of color. And as you can see, she's not.

NICKISCH: At a Brown campaign stop over the weekend, supporters were filmed doing the tomahawk chop.

(SOUNDBITE OF CHANTING)

NICKISCH: Senator Brown later said he doesn't condone that but that the real offense is that Elizabeth Warren claimed minority status. She is sticking to the story. She says her family told her growing up that her mother was part Native American.

(SOUNDBITE OF DEBATE)

ELIZABETH WARREN: But I knew my father's family didn't like that she was part Cherokee and part Delaware. So my parents had to elope.

NICKISCH: After Scott Brown took his charge to the airwaves this week, Warren responded within hours.

(SOUNDBITE OF TV AD)

WARREN: Let me be clear. I never asked for, never got any benefit because of my heritage. The people who hired me have all said they didn't even know about it. I'm Elizabeth Warren. I approve this message. Scott Brown can continue attacking my family. But I'm going to keep fighting for yours.

DR. TOBE BERKOVITZ: The voters liked him as sort of their friendly neighbor. And now, if he is a politician sort of wielding a knife, that is not going to be good for his image.

NICKISCH: Brown's image as a regular guy who drives a pickup truck helped him win a stunning upset two years ago.

MICHAEL MCGRATH: Personally, he seems like a nice guy, seems like a good guy. You know, the kind of guy you'd hang out with,

NICKISCH: Michael McGrath is a Democrat, and he's going to vote for Warren. But so far, he's not blaming Senator Brown for going negative.

MCGRATH: It's a political race, anything goes. I mean, it's the nature of the beast.

NICKISCH: Another voter, Joan Hoffman, is a Brown supporter. She points out that Elizabeth Warren aired the first attack ad in this race.

JOAN HOFFMAN: If he continues to play the good guy and she keeps throwing these barbs at him, he's going to show a sign of weakness. He does have to defend himself.

NICKISCH: Still, if Brown continues to launch personal attacks against the woman running again him for the next six weeks, voters may not be so forgiving, especially in a race where both candidates signed what they called the People's Pledge to keep often vicious third-party ads out. But Republican political analyst Todd Domke says it's not stopping the negativity.

TODD DOMKE: When the People's Pledge was first announced, people had higher expectations about this being an issues kind of contest. But now it will look more like a typical campaign.

NICKISCH: The agreement may not change the tone of the race. But Domke says at least it holds the candidates accountable for it. For NPR News, I'm Curt Nickisch in Boston. Transcript provided by NPR, Copyright NPR.